A rotary loop taker is a device that must be incorporated into all lock-stitch sewing machines. Perhaps 70 to 80 percent or more of all industrial sewing machines are of the lock-stitch type, and therefore utilize a rotary loop taker. Lock-stitch sewing machines of the type described are especially useful for sewing light-weight canvas or leather, or other light to medium-weight materials.
A conventional loop taker is cast or forged of fine steel and precision machined to exact proportions and balance throughout its extent from its weighted hub to its fine hook or "loop seizing point." It is a costly item, and a short-lived item under the heavy wear and tear that accompanies the use of a typical industrial sewing machine. Conventional loop takers normally have a life of only three to six months, depending on the many variables involved.
The loop taker and bobbin assembly operates under very demanding conditions. The loop taker typically rotates at 8,000 to 16,000 revolutions per minute around a stationary bobbin basket held by a bearing rib on its exterior circumference which rides in a circumferential raceway on the interior of the rotary loop taker frame. The bearing rib and the raceway are subjected to considerable vibration and impact and tend to become flawed, especially at their leading edges and initial portions. A flawed bearing rib or raceway quickly becomes seriously defective. The thread is then unable to pass freely around the bobbin case and often becomes jammed or broken, which necessitates replacement of the loop taker.
The most vulnerable part of the fragile loop seizing point of a rotary loop taker is the free end of the point. The tip can, for example, be chipped by the needle of the sewing machine or burred by the friction that is created by the high speed revolutions of the loop taker as it picks up the thread off the needle. Since a faulty hook or loop seizing point tends to skip stitches, fray or break thread, it must be repaired or replaced whenever its fragile loop seizing point accidentally breaks or becomes too dull through normal wear.
With a rotary loop taker of the usual type, most factories simply discard the entire device when the loop seizing point (which as pointed out above is conventionally an integrally formed part of the loop taker) becomes chipped or otherwise rendered unusable. Others send the rotary loop taker to a facility that reprocesses the tip of the loop seizing point. Either solution is very costly.
The common approach to reducing the expense of replacing or refurbishing worn-out loop takers has been to attempt the design of a detachable, replaceable loop seizing point. Under that approach, the loop seizing point is replaced, while the remainder of the loop taker is saved. Beginning at least as early as 1922, there has been a discussion in the industry of the need to reduce replacement costs by designing such a replaceable loop seizing point. See Dickson U.S. Pat. No. 1,431,380, issued Oct. 10, 1922. A number of other inventors followed the same approach, including Corral et. al. U.S. Pat. No. 2,002,172 issued May 21, 1935, Joseph U.S. Pat. No. 2,495,637 issued Jan. 24, 1950, Thierman German Pat. No. 933,601 issued Sept. 29, 1955, Grabowski U.S. Pat. No. 3,139,050 issued June 30, 1964, Corey U S. Pat. No. 3,140,681 issued July 14, 1964, Corey U.S. Pat. No. 3,223,060 issued Dec. 14, 1965 and Kuhar U.S. Pat. No. 3,465,700 issued Sept. 9, 1969. These devices had little commercial success, mainly because most of them described devices with laminations parallel to the axis of rotation of the rotary loop taker shaft, which created cracks and discontinuities where dirt or lint would collect, the thread would catch, or the assembly would be weakened.
The problem of expensive loop taker replacement was finally overcome with the detachable loop seizing point described in Badillo U.S. Pat. No. 4,493,278 issued Jan. 15, 1985, assigned to the assignee of the present invention. The Badillo patent successfully described a detachable loop seizing point that utilizes an inwardly extending lug foot to avoid the cracks and discontinuities pervasive in earlier attempts.
Conventional one-piece metal loop takers suffer from several other drawbacks aside from costs, and those drawbacks are generally shared by two-piece designs with detachable loop seizing points. For example, the loop takers require frequent oiling of the contact surfaces between the loop taker and the bobbin basket. The oil occasionally finds its way to the thread and is carried onto the cloth. A significant number of rejections result.
Another problem inherent in metal loop takers is galling of the metal surfaces in contact with one another. High speed operation of the loop taker creates heating and friction between the bobbin basket bearing rib and the frame raceway, which can ultimately cause the bearing surfaces to wear out, overheat or "freeze up." This is a particular problem in machines used for sewing fine cloth which are oiled sparingly to avoid oil stains.
Still another troublesome condition that results from the wearing of the bobbin case raceway in the conventional rotary loop taker is known in the industry as "slop." This condition is the excessive "play" between the bobbin basket and the inner wall of the loop taker which defines the bobbin case raceway. Slop may cause skipping of stitches because the needle may not enter the proper location through the bobbin basket in horizontal shaft machines to form a small loop to be picked up by the seizing point. Slop interferes with the proper release of the top thread (i.e., the needle thread) from around the bobbin case, and increases the incidence of jamming between the bobbin case and raceway. It also tends to cause large, undesirable loops of top thread to be formed on the bottom of the material being sewed, because of the premature closing of the escape exit for the top thread. It may also cause the top thread to break, if a bunching of thread occurs because of the degree of "slop" that is present. Finally, if serious jamming of the top thread occurs, the upper ledge of the bobbin case raceway on the loop seizing point may be broken as the operator or mechanic manipulates the bobbin basket in an attempt to free up the jammed thread.
Even loop takers made of expensive, high strength, precision machined steel have short lives under these demanding conditions. Thus, it is no surprise that the industry has assumed that effective loop takers of inexpensive, easily machined materials would not be possible. However, that is exactly the approach of the present invention. The inventions described above sought to reduce expenses by permitting replacement of only the loop seizing point while saving the rest of the loop taker. In contrast, this invention reduces expenses by altering the composition of the loop taker and thereby reducing its manufacturing costs. The reduced manufacturing costs are achieved by utilizing a composite of metal on high wear and high impact areas, embedded in a novel frame of plastic or other flowable material that is easily manufactured without expensive machining, forging or casting. Thus, the loop taker is still discarded after the loop seizing point wears out, but the replacement cost is dramatically lower. Not only is the present invention much less expensive to manufacture than existing loop takers, it is also surprisingly more effective. As described in detail below, the plastic surfaces are lightweight, self-lubricating and nearly friction-free.
A loop taker of two dissimilar materials is described in Haas U.S. Pat. No. 2,219,308 issued Oct. 29, 1940, but that loop taker is for a specialized application not relevant to this invention. The Haas invention is for a low speed wax thread sewing machine. As explained in that patent, loop takers of wax thread sewing machines must be kept at a constant temperature to keep the wax liquid without burning it. The invention utilizes certain resin components, but primarily for the purpose of promoting even heat distribution. Haas also recognized that a wax thread loop taker must have smooth continuous thread-bearing surfaces to avoid catching the thread or accumulating wax on the loop taker. However, the approach of Haas for achieving smooth surfaces was exactly the opposite of the approach of the present invention, and in fact teaches away from the present invention. Haas used resin components on the thread bearing surfaces with optional interior metal reinforcing members. On the other hand, the applicant of the present invention takes care to avoid any non-metal thread bearing surfaces, since these surfaces are subjected to high surface stress and impact. While Haas might have been feasible for low speed wax thread (although there is no evidence the invention was actually commercialized or successful), it would not have survived the conditions of high speed dry thread machines.